1. Field of the Invention
The present invention relates generally to the fields of developmental biology and molecular biology. More particularly, it concerns the proteins, classified here as MCIPs, identified as regulating calcineurin function.
2. Description of Related Art
Calcineurin is a serine/threonine protein phosphatase that plays a pivotal role in developmental and homeostatic regulation of a wide variety of cell types (Klee et al., 1998; Crabteee, 1999). The interaction of calcineurin with transcription factors of the NFAT family following activation of the T cell receptor in leukocytes provides the best characterized example of how calcineurin regulates gene expression (Rao et al., 1997). Changes in intracellular calcium promote binding of Ca2+/calmodulin to the catalytic subunit of calcineurin (CnA), thereby displacing an autoinhibitory region and allowing access of protein substrates to the catalytic domain. Dephosphorylation of NFAT by activated calcineurin promotes its translocation from the cytoplasm to the nucleus, where NFAT binds DNA cooperatively with an AP1 heterodimer to activate transcription of genes encoding cytokines such as IL-2. This basic model of NFAT activation has been shown to transduce Ca2+ signals via calcineurin in many cell types and to control transcription of diverse sets of target genes unique to each cellular environment (Timmerman et al, 1996). In each case, NFAT acts cooperatively with other transcription factors that include proteins of the API (Rao et al., 1997), cMAF (Ho et al., 1996), GATA (Mesaeli et al., 1999; Molkentin et al., 1998; Musaro et al., 1999), or MEF2 (Chin et al., 1998; Liu et al., 1997; Mao et al., 1999; Mao and Wiedmann, 1999) families. In addition to T cell activation, cellular responses controlled by calcineurin signaling include synaptic plasticity (Mao et al., 1999; Graef et al., 1999; Zhuo et al., 1999) and apoptosis (Wang et al., 1999; Youn et al., 1999).
Recent studies of calcineurin signaling in striated myocytes of heart and skeletal muscle have expanded the scope of important physiological and pathological events controlled by this ubiquitously expressed protein. Forced expression of a constitutively active form of calcineurin in hearts of transgenic mice promotes cardiac hypertrophy that progresses to dilated cardiomyopathy, heart failure, and death, in a manner that recapitulates features of human disease (Molkentin et al., 1998). Moreover, hypertrophy and heart failure in these animals, and in certain other animal models of cardiomyopathy, are prevented by administration of the calcineurin antagonist drugs cyclosporin A or FK-506 (Sussman et al., 1998). In skeletal muscles, calcineurin signaling is implicated both in hypertrophic growth stimulated by insulin-like growth factor-1 (Musaro et al., 1999; Semsarian et al., 1999), and in the control of specialized programs of gene expression that establish distinctive myofiber subtypes (Chin et al., 1998; Dunn et al., 1999). These observations have stimulated interest in the therapeutic potential of modifying calcineurin activity selectively in muscle cells while avoiding unwanted consequences of altered calcineurin signaling in other cell types (Sigal et al., 1991).
The activity of calcineurin in mammalian cells can be modulated by interactions with other proteins. These include not only immunophilins that are the targets of the immunosuppresant drugs cyclosporin A and FK-506, but two unrelated proteins (AKAP79 and cabin-1/cain) that were identified recently. AKAP79 binds calcineurin in conjunction with protein kinase C and protein kinase A, serving as a scaffold for assembly of a large hetero-oligomeric signaling complex (Kashishian et al., 1998). Cabin-1/cain binds both calcineurin and the transcription factor MEF2 (Sun et al., 1998; Lai et al., 1998). As a consequence of cabin-1 overexpression, calcineurin activity is inhibited and MEF2 is sequestered in an inactive state. Another calcineurin-binding protein is Rex1p (YKL159c) of Saccharomyces cerevisiae. A preliminary report noted that this small 24 kDa protein inhibits calcineurin signaling when overexpressed in yeast (Kingsbury and Cunningham, 1998).
The existence of calcineurin regulating proteins gives rise to potential therapeutic interventions targeting these molecules. Identifying new, more suitable candidates having the ability to modulate calcineurin function in cardiac tissue is an important goal of current research efforts.